Modeling and control of a grid-connected small-scale windmill system using a pulse width modulated modular multilevel converter

Topologies based on the modular multilevel converter (MMC) represent an attractive alternative against traditional two-level voltage source converter as the interphase between renewables and the ac grid. MMCs feature lower losses, improved voltage quality, scalability and redundancy. In this paper, a pulse width modulated MMC-based topology is proposed for the grid-interconnection of a small-scale wind energy conversion system (WECS). The WECS is realized through the cascaded connection of a wind turbine, a permanent magnet synchronous generator (PMSG), two back-to-back MMCs inter-phased through a dc-link capacitor, an LC-filter, and a coupling transformer. Mathematical modeling of the various components is developed throughout the paper, along with a control strategy able to perform both maximum power point tracking and arbitrary reactive power injection. The approach is validated via detailed PSCAD/EMTDC computer simulations using real wind speed data.